A. Technical Challenges to the Preparation of Thermoplastic Nanocomposites
While thus having many potential benefits, the incorporation of a well-dispersed nanofiller into a polymer matrix is usually quite challenging.
The most common difficulty that is encountered in preparing polymer nanocomposites involves the need to disperse the nanofiller. The specific details of the source and severity of the difficulty, and of the methods that may help overcome the difficulty, differ between types of nanofillers, polymers, and fabrication processes (for example, the “in situ” synthesis of the polymer in an aqueous or organic medium containing the nanofiller, versus the addition of the nanofiller into a molten polymer). However, some important common aspects can be identified. Most importantly, nanofiller particles of the same kind often have strong attractive interactions with each other. As a result, they tend to “clump together”; for example, preferably into agglomerates (if the nanofiller is particulate), bundles (if the nanofiller is fibrous), or stacks (if the nanofiller is discoidal). In most systems, their attractive interactions with each other are stronger than their interactions with the molecules constituting the dispersing medium, so that their dispersion is thermodynamically disfavored and hence extremely difficult. Even in systems where the dispersion of the nanofillers is thermodynamically favored, it is often still very difficult to achieve because of the large kinetic barriers (activation energies) that must be surmounted. Consequently, nanofillers are very rarely easy to disperse in a polymer. A common strategy is to (a) apply a large amount of energy (as in a vigorous shaking, shearing, and/or milling process) to overcome the kinetic barriers so that the nanofiller clumps can be broken apart, and (b) incorporate additives (such as dispersants and/or coupling agents) into the starting formulation to stabilize the nanofiller dispersion thermodynamically once it is achieved.
Another difficulty with the fabrication of nanocomposites is the fact that, once the nanofiller is dispersed in the appropriate medium (for example, an aqueous or organic medium containing the nanofiller for the “in situ” synthesis of the polymer, or a molten polymer into which nanofiller is added during melt compounding), the viscosity of the resulting dispersion may (and often does) become very high. When this happens, it can impede the successful execution of the fabrication process steps that must follow the dispersion of the nanofiller to complete the preparation of the nanocomposite. Dispersion rheology is a vast area of both fundamental and applied research. It dates back to the 19th century, so that there is a vast collection of data and a good fundamental understanding of the factors controlling the viscosities of dispersions. Nonetheless, it is still at the frontiers of materials science, so that major new experimental and theoretical progress is continuing to be made. In fact, the advent of nanotechnology, and the frequent emergence of high dispersion viscosity as an obstacle to the fabrication of polymer nanocomposites, have been instrumental in advancing the state of the art in this field. Bicerano, et al. (1999) have provided a comprehensive overview which can serve as a resource for workers interested in learning more about this topic.
Melt compounding is a commonly practiced prior art technique for preparing both conventional composites and nanocomposites of styrenic thermoplastic polymers. As of the date of this disclosure, an excellent concise summary of this technique is provided on the website of the RTP Company which is a leading thermoplastic compounder: “Compounding is a process of melt blending plastics with other additives. This process changes the physical, thermal, electrical or aesthetic characteristics of the plastic. The final product is called a compound or composite. Compounding starts with a base resin or polymer . . . . By incorporating an extensive range of additives, fillers, and reinforcers, a wide range of properties can be achieved in conductivity, flame retardance, wear resistance, structural, and precolored . . . . . For example, glass fibers can be added at various levels to increase stiffness in a resin that is more flexible than desired. Compounding is done in several steps. Resin and additive(s) are fed through an extruder where they are combined. The melted compound exits the extruder in strands about the diameter of yarn. These strands are cooled and cut into pellets.” An additional potential difficulty may be encountered in systems where chemical reactions (such as “in situ” polymerization) are taking place in a medium containing nanofiller, for example during the use of a process such as suspension polymerization starting from a formulation containing both monomers and a nanofiller to manufacture polymeric nanocomposite beads. [General background information about suspension polymerization can be found in the review articles by Brooks (2010) and Pinto et al. (2013). U.S. Pat. Nos. 7,803,740, 7,803,741, and 7,803,742 provide representative examples of the use of suspension polymerization to manufacture styrenic thermoset nanocomposite beads for use in various oil and natural gas drilling applications.] This is the possibility that the nanofiller may have an adverse effect on the chemical reactions, and thus for example prevent the growth of polymer chains whose average molecular weights are sufficiently large to provide acceptable mechanical properties. (In the case of polymerizing formulations containing crosslinking monomers and thus leading to the synthesis of thermoset instead of thermoplastic polymers, in addition to polymer chain growth disruption, network formation can also be disrupted. On the other hand, the challenge of achieving an optimum molecular distribution of polymer chains of finite degree of polymerization, so that articles can be fabricated without difficulty from the polymer by melt processing and the fabricated articles manifest acceptable mechanical properties is a special challenge in the case of thermoplastic polymers. This special challenge does not exist in the case of thermoset polymers which possess continuous three-dimensional network architecture and which are hence not subjected to melt processing.) The combined consideration of the work of Lipatov, et al. (1966, 1968), Popov, et al. (1982), and Bryk, et al. (1985, 1986, 1988) helps in providing a broad perspective into the nature of the difficulties that may arise. It can be seen that such disruptive effects can arise from various root causes. For example, the presence of a filler with a high specific surface area can disrupt “in situ” polymerization in a process such as the suspension polymerization of polystyrene nanocomposite. Such an outcome can arise from the combined effects of the adsorption of initiators on the surfaces of the nanofiller particles and the interactions of the growing polymer chains with the nanofiller surfaces. Adsorption on the nanofiller surface can affect the rate of thermal decomposition of the initiator. Interactions of the growing polymer chains with the nanofiller surfaces can result both in the reduction of the mobility of growing polymer chains and in their breakage. Very strong attractions between the initiator and the nanofiller surfaces (for example, the grafting of the initiators on the nanofiller surfaces) can potentially augment all of these detrimental effects.
Thus it is clear that the development of new and improved formulations and processes for manufacturing thermoplastic nanocomposites is a continuing need of industry with a broad range of applications for inventions that can provide some of the potential benefits of nanofiller incorporation while also providing a means for overcoming some of the technical challenges.